<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
		<script type="text/javascript">
			// null专门表示空对象
			var a=null;
			console.log(typeof a)
			// undefined未赋值的变量
			var b;
			console.log(b)
			console.log(typeof b)
		</script>
	</head>
	<body>
	</body>
</html>
